This invention relates to a liquid crystal displaying apparatus applied to projectors and projection-type displaying apparatuses of various sizes, such as view finders, personal computers and large screen televisions.
Apparatuses such as flat panel displays and video projectors with liquid crystal displays have been developed. And, there has been increased demand for video projectors to display images on a large screen.
Video projectors are roughly classified into two types, that is, a transmission type and a reflection type. The transmission-type video projector is provided with a transmission-type active-matrix liquid crystal display. And, the reflection-type video projector is provided with a reflection-type active-matrix liquid crystal display.
FIG. 1 illustrates an active-matrix liquid crystal display device in a sectional view corresponding to one pixel.
The active-matrix liquid crystal display includes a MOSFET 2 and a charge storage capacitor 3 formed on a silicon substrate 1. The MOSFET 2 consists of a drain 5, a gate 6 and a source 7. An Al reflecting electrode layer 8 is formed over the MOSFET 2 via an insulating layer 4. A lower portion of the reflecting electrode layer 8 is connected to the source 7 of the MOSFET 2. A flat signal detecting section 9 is extended from the connecting portion toward the lateral direction. The signal detecting section 9 and an insulating film 10 interposed between the signal detecting section 9 and the substrate 1 constitute the charge storage capacity 3.
The MOSFET 2 serving as a switching element and the charge storage capacitor portion 3 for one pixel constitute an active element circuit with respect to the substrate 1. An active element substrate 11 is constituted as a whole.
A transparent common electrode film 23 is formed on one side of a glass substrate 22 to constitute a transparent substrate 21.
The insulating layer 4 and the reflection electrode layer 8 are covered by an orientation film 12. And, the transparent common electrode film 23 is covered by an orientation film 24. A liquid crystal layer 25 is put between the orientation films 12 and 24 and is hermetically sealed therebetween.
The operation of the liquid crystal display shown in FIG. 1 will be described with reference to the equivalent circuit diagram of FIG. 2.
As shown in FIG. 2, a gate line Xj and a signal line Yj are connected to the gate 6 and the drain 5, respectively, of each MOSFET 2. The gate line Xj and the signal line Yj supply a select signal and a video signal to the gate 6 and the drain 5, respectively.
The MOSFET 2 are turned on when the select signal from an X-address circuit is applied to each gate 6 through the gate line Xj. The video signal is then supplied to the reflection electrode layer 8 through the drain 5 and the source 7 to charge the charge storage capacitor 3 through the signal detection section 9.
The reflection electrode layer 8 will be held at a specific potential for a period of time determined by the time constant decided by the capacity and the discharge resistance of charges stored in the charge storage capacitor 3 even if the select signal on the gate line Xj is set at zero level.
A voltage generated across the reflection electrode layer 8 and the common electrode film 23 is applied to the liquid crystal layer 25 for the period of time determined by the time constant to reorient the liquid crystal molecules under an electric field generated by the applied voltage. This liquid crystal molecule reorientation controls polarization of light transmitted through the liquid crystal layer 25. At the same time, the voltage applied to the liquid crystal layer 25 is controlled with a video signal from a Y-address circuit supplied on the signal line Yj to modulate light beams which are incident (as read light) to the glass substrate 22, reflected by the reflection electrode layer 8 and emitted (as reflected light) from the glass substrate 22.
Practically, a select signal is supplied to the gate line Xj to turn on all the MOSFETs 2 connected to the gate line Xj. Then, each charge storage capacitors 3 of these MOSFETs 2 is charged with a video signal supplied to a signal line Yj, thereby modulating the incident light beams per pixel while being reflected from the reflection electrode layer 8.
Japanese Patent Application No. 294453/1996 discloses, in FIG. 3, a reflection-type color liquid crystal displaying apparatus with the liquid crystal display explained above.
In FIG. 3, laminated on a reflection-type liquid crystal display device 31 are a thin plate glass layer 32, a color filter 33, a glass substrate 34, and a coupling prism 35. The color filter 33 is made of a transmission-type hologram with laminated holographic lens array layers 33r, 33g and 33b corresponding to three primary colors of R, G and B, respectively. The lens array layers 33r, 33g and 33b included holographic lenses 33re, 33ge and 33be, respectively. The coupling prism 35 is made of a flat glass plate with one end surface and the upper surface. The end surface is formed so that read light beams obliquely incident to the color liquid crystal displaying apparatus are vertically incident thereto. The read light beams incident to the color liquid crystal displaying apparatus via the end surface are modulated as described later and emitted therefrom via the upper surface.
In detail, the read light beams are incident to the color filter 33 via the coupling prism 35 and the glass substrate 34. The holographic lenses 33re, 33ge and 33be diffract only S-polarized light components of the incident light beams for each primary color. The S-polarized light components diffracted for each primary color are vertically focused onto pixel electrodes 8r, 8g and 8b of a liquid crystal display device 31, respectively.
On the other hand, P-polarized light components of the incident light beams are transmitted through the color filter 33, reflected by the reflection electrode layer 8, and emitted outside the color liquid crystal displaying apparatus.
The liquid crystal display device 31 has basically the same structure as the device shown in FIG. 1. The thin plate glass layer 32 corresponds to the glass substrate 22 of FIG. 1. The device 31 is provided with a dielectric mirror film 26 to achieve high reflection factor, which is interposed between the orientation film 12 and the reflection electrode layer 8.
The pixel electrodes 8r, 8g and 8b of the reflection electrode layer 8 are arranged into a mosaic or stripe. Voltages for one pixel to be displayed are applied to one group of pixel electrodes 8r, 8g and 8b based on a video signal. Because pixels for the one group of pixel electrodes constitute the one pixel to be displayed.
Under the configuration, the S-polarized light components diffracted for the primary colors are focused on the pixel electrodes 8r, 8g and 8b and reflected by the dielectric mirror film 26. The reflected S-polarized light components are incident again to the liquid crystal layer 25 and also to the color filter 33. The reflected S-polarized light components incident to the liquid crystal layer 25 are converted into P-polarized light components according to a modulation factor decided by the video signal when transmitted through the liquid crystals. Because the liquid crystals have been reoriented for each pixel by an electric field generated across the pixel electrodes 8r, 8g and 8b, and the common electrode film 23.
The P-polarized light components converted by the liquid crystal layer 25 are transmitted through the color filter 33 and emitted from the coupling prism 35 through its upper surface. On the other hand, the S-polarized light component are diffracted by the color filter 33 and returned to the direction from which the read light beams have been incident to the color liquid crystal displaying apparatus because the color filter 33 diffracts S-polarized light components only.
The P-polarized light components are then transmitted through a polarization plate 36 provided on the upper surface of the coupling prism 35. The P-polarized light components are further transmitted to an optical system and projected on a screen. The polarization plate 36 is provided such that its transmission axis is set in a direction in which only the P-polarized light components are allowed to be transmitted therethrough.
The color liquid crystal displaying apparatus using modulated P-polarized light components as projection light beams is provided with the polarization plate 36 to prevent decrease in contrast of a color image to be displayed. The decrease in contrast will occur if the polarization plate 36 is not employed because the color filter 33 allows not only the P-polarized light components but also the S-polarized light components to pass therethrough.
Next, FIG. 4 illustrates another color liquid crystal displaying apparatus with a transmission-type liquid crystal display.
The color liquid crystal displaying apparatus is provided with two polarization plates 41 and 44. Laminated therebetween are a color filter 42 made of a transmission-type hologram, and a transmission-type liquid crystal display 43.
The transmission-type apparatus employs the liquid crystal display 43 the substrate and pixels electrodes of which are made of transparent materials, and hence no dielectric mirror film is provided.
The color filter 42 is capable of light diffraction and focusing, however, not conversion of polarization of read light vertically incident thereto.
When read light beams are incident to the polarization plate 41 in advance, only the S-polarized light components of the read light beams are transmitted to the color filter 42. The S-polarized light components are diffracted by the color filter 42 into S-polarized light components corresponding to primary colors R, G and B and focused on the corresponding pixel electrodes of the liquid crystal display 43.
The diffracted S-polarized light components that pass through the pixel electrodes are incident to a liquid crystal layer and converted into P-polarized light components according to a modulation factor decided by a video signal when transmitted through the liquid crystals. Because the liquid crystals have been reoriented for each pixel by the pixel electrodes.
As described above, a color liquid crystal display apparatus is a combination of a liquid crystal display and a color filter. And, the liquid crystal display includes vertical orientation mode-liquid crystals, such as nematic liquid crystals with negative dielectric anisotropy to achieve high contrast ratio in image projection. The nematic liquid crystals are controlled such that liquid crystal molecules aligned in a direction perpendicular to the substrate is reoriented by a voltage applied thereto. The vertical orientation mode-liquid crystals exhibit sharp variation in light transmission factor to applied voltage, or sharp threshold characteristics, to achieve an image of high contrast ratio. In this regard, vertical orientation mode-liquid crystals with a visual field angle of 140.degree. or more and a response time of 25 ms or less have been developed.
In liquid crystal display, a narrow pixel electrode pitch achieves high resolution of image, however, this causes a lateral electric field between adjacent pixels and which affects liquid crystals as shown in FIG. 5. The vertical orientation mode-liquid crystal display is apt to undergo its influence, that is, orientation of liquid crystal molecules are disturbed to cause disclination.
In other words, white information displaying will not generate a lateral electric field between pixel electrodes because the same level voltages are applied to R, G and B pixels. On the other hand, Ye (yellow) displaying will generate a lateral electric field because a voltage applied to a B pixel electrode is lower than those applied to R and G pixel electrodes. This results in disclination which affects modulation in an area where liquid crystals are subjected to the disclination. This causes low luminance, color balance, color purity, and so on.
Generally, disclination can be prevented by rubbing on the orientation film to have a large pretilt angle of liquid crystals to limit reorientation for prevention of disclination. However, a large pretilt angle in vertical orientation mode-liquid crystal will cause a relatively large double refraction on liquid crystal molecules even under no electric field. This results in low contrast ratio. In other words, there exists a contradictory relationship between contrast ratio and color reproducibility by preventing disclination.
In order to solve this problem, the applicant of this application has proposed a liquid crystal display (in Japanese Patent Application No. 334863/1996) in which a pretilt angle .alpha..sub.1 given by an orientation film 12 of active element substrate 11 side is set larger than an pretilt angle .alpha..sub.2 given by orientation film 24 of transparent substrate 21 side as shown in FIG. 6.
This liquid crystal display achieves prevention of disclination without lowering contrast ratio by limiting reorientation liquid crystal molecules in the vicinity of the active element substrate 11 where a large lateral electric field is generated and making small the pretilt angle .alpha..sub.2 in the vicinity of the transparent substrate 21 where a small lateral electric field is generated. However, the pretilt angle .alpha..sub.1 must be set at 4 degrees or more, and hence no sufficiently high contrast ratio is still achieved.